1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device, and more particularly, to an organic EL device having improved structures of electrode patterns and a method of manufacturing the same.
2. Description of the Related Art
Recently, much attention has been paid to EL devices because they have advantageous features suitable for next generation devices, such as a wide viewing angle, a high contrast ratio and a high response speed. EL devices are classified into inorganic EL devices and organic EL devices according to materials to form emitter layers.
In particular, studies of organic EL devices have been briskly carried out because of their advantages, including good characteristics in terms of brightness and response speed, color displaying and so on.
An EL device is basically configured such that an anode is formed on a transparent insulating substrate, e.g., a glass substrate, in a predetermined pattern, a light emitter layer consisting of organic or inorganic layers is formed on the anode, and a cathode having a predetermined pattern is then stacked thereon so as to be orthogonal with the anode.
The organic or inorganic layers have a layered structure of a hole transport layer, a light emitting layer and an electron transport layer sequentially stacked. As described above, the light emitting layer is made of either an organic or inorganic material.
Usable materials of the organic layer include copper phthalocyanine (CuPc), N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) and tris-8-hydroxyquinoline aluminum (Alq3).
In the above-described EL device, when a drive voltage is applied to the anode and the cathode, holes from the anode migrate to the light emitting layer via the hole transport layer and electrons from the cathode migrate to the light emitting layer via the electron transport layer. The holes and the electrons are recombined in the light emitting layer to generate excitons. As the excitons are deactivated to a ground state, fluorescent molecules of the light emitting layer emit light, thereby forming an image.
As described above, EL devices are classified into organic EL devices and inorganic EL devices according to materials for light emitting layers. An explanation will now be given by referring to an organic EL device.
In an organic EL device, an anode formed on the top surface of a substrate in a predetermined pattern is a transparent electrode, e.g., indium tin oxide (ITO), which is ordinarily formed by photolithography. Organic layers are formed on the anode by vacuum deposition and a cathode is then patterned thereon.
In the organic EL device having the above-described configuration, a high precision degree is required in patterning the light emitting layer of the organic layers and patterning a cathode corresponding to an anode, and a variety of techniques have been proposed.
One of typical methods of forming a cathode having a predetermined pattern is photolithography. If the cathode is formed by photolithography, there is, however, a problem in that, when the cathode is selectively etched using a photoresist, the photoresist is stripped off or development is carried out, moisture may soak into an interface between organic layers and the cathode, resulting in degradation in electroluminescent performance due to deteriorated characteristics of the organic layer, a low luminous efficiency, and a shortened life due to stripping of the cathode.
For overcoming those problems arising due to moisture ingress, a new method using vacuum deposition in which an evaporation mask is employed has been utilized to form organic layers or cathodes. However, it is quite difficult to form fine patterns on a large substrate using an evaporation mask. Currently, studies of vacuum deposition to form finer patterns are being briskly carried out.
Recently known methods to solve the above-described problems include vacuum deposition using a cathode separator as disclosed in U.S. Pat. No. 5,701,055, which is illustrated in FIG. 1. This will now be described in detail.
First, an anode 2 is patterned on a transparent substrate 1, and cathode separators 5 whose top portions are wider than the bottom portions are formed thereon so as to be arranged perpendicular to the anode 2. Organic layers 3 are formed between each of the cathode separators 5 and cathodes 4 are blanket-deposited thereon, thereby attaining the same effect as patterning the cathodes 4 on portions where the cathode separators 5 are not formed.
In the above-described configuration, in order to provide a cathode patterning effect by the cathode separators 5, the top portions of the cathode separators 5 must be wider than the bottom portions thereof. Also, the organic layers 3 are formed on the anode 2 and the cathodes 4 are formed to be narrower than the organic layers 3, thereby preventing the short-circuit between the anode 2 and the cathodes 4 due to the organic layers 3. According to this method using the shade of a separator, however, highly accurate patterning and high-speed deposition cannot be achieved under the conditions of variable deposition rates and a large amount of material deposited. Also, since portions where the cathodes 4 are not formed corresponding to the shade exist in the organic layers 3, these portions are susceptible to ingress of moisture, resulting in deterioration of the organic layers 3. The deterioration of the organic layers 3 may cause the short-circuit of the anode 2 and the cathodes 4.
Another method of using cathode separators recently having been used in depositing cathodes utilizes a deposition mask such that slits of the deposition mask are arranged between the cathode separators 5 illustrated in FIG. 1 for cathode deposition. In this case, however, there is a problem in that the cathode separators collapse due to vibration of the deposition mask, resulting in the short-circuit. Korean Patent Application Publication No. 2000-60589 discloses a technology to overcome this problem, that is, auxiliary barriers higher than the cathode separators are provided between the cathode separators.
In another method of forming cathodes, patterned cathodes are directly formed using a deposition mask. This method also has many problems, that is, the thin slit-shaped deposition mask may experience a sag of its central portion in a larger substrate, causing damage to organic layers or cathodes, thereby adversely affecting the yield. The sag also makes it impossible to form cathodes having finer patterns.
A known technique to solve the sag problem of a deposition mask includes disposing a magnetic medium at the opposite side of the deposition mask and closely contacting the deposition mask with an organic layer. However, close contact of the deposition mask may cause the organic layer to be damaged, resulting in the short-circuit between an anode and a cathode.
In order to prevent the damage of an organic layer due to a deposition mask, Japanese Patent Laid-Open Publication No. hei 10-241859 discloses an organic electroluminescent device having partition walls 6 with a predetermined height formed between the respective lines of an anode 2′, as illustrated in FIG. 2A. That is, the partition walls 6 with a predetermined height are formed between lines of the anode 2′ with a predetermined pattern on a transparent substrate 1′, and an organic layer 3′ and a cathode 4′ are then formed on the substrate 1′ having the partition walls 6. In forming the cathode 4′ using a deposition mask, a shielding portion of the deposition mask cannot directly damage the organic layer 3′ constituting pixels by the partition walls 6, thereby preventing the short-circuit between the anode and the cathode due to damage to the organic layer 3′.
However, according to this method, a gap (G) between the anode 2′ and each of the partition walls 6 makes the organic layer 3′ formed at the edge of the anode 2′ thinner, as illustrated in FIG. 2B. As a result, the cathode 4′ may contact the anode 2′ at a lateral portion 7 of the anode 2′, causing the short-circuit between the cathode 4′ and the anode 2′.